Nuclear reactors cooled by a liquid metal such as sodium are well known, and the circulating hot liquid metal coolant has been utilized for generating power by heat transfer from the liquid metal to water, which in turn is converted to high pressure steam. The steam is then cycled to a turbine-generator power conversion system for generating electricity.
A major drawback and a safety problem in such steam generators is the need to protect the system against the violent metal-water reactions that may result from a leak in the liquid metal and/or water circulation systems. Should the liquid metal reactor coolant come into direct contact with steam or water leaking out from the steam generator tube, a violent chemical reaction occurs with a corrosive byproduct (e.g., NaOH) and free hydrogen. Conventional reactor-power plant systems employ an intermediate liquid metal heat exchange circuit to protect the reactor core in the event of a leak. Typically, such an intermediate system includes an expansion vessel, complex piping circuits, a heat exchanger, a pump, liquid metal purification equipment, fill and drain systems, electrical preheat systems, and the attendant instruments, controls and structures for housing and support of these components.
From the standpoint of efficiency, design simplicity and conservation of physical space and other resources it would be highly advantageous to eliminate such intermediate systems, however a steam generator design of exceptional reliability or with special protective features such as a double tube wall design would be required.
A drawback of known double tube steam generator systems is their inefficiency in transferring heat from the liquid metal coolant to water. Prior art steam generators of double wall construction have relied on inert gas as a heat transfer medium, however an inert gas barrier is extremely inefficient for this purpose. U.S. Pat. Nos. 3,545,412, 3,613,780 and 3,907,026, for example, show apparatuses wherein closely placed tubes containing liquid metal or water are surrounded by inert gas, or wherein water tubes are run through a sleeve containing inert gas separating the water and liquid metal coolant. Other prior art duplex tube steam generators have used bonded tubes or duplex tubes with mercury as the intermediate heat transfer agent. Bonded tubes can experience difficulties associated with loss of contact stress due to thermal aging. Duplex tubes with mercury pose a safety problem for the reactor core, because typical liquid metal coolants, i.e., sodium, react with the mercury to form an amalgam.
Furthermore, conventional steam generators are large and bulky due to use, typically, of straight tube design. As a result, integration of a steam generating system with the reactor is often complex and costly. Furthermore, such steam generator designs present difficulties in locating a failed tube and in accomodating tube-to-tube and tube-to-shell temperature gradients.
Conventional steam generator systems are also characterized by fabrication and repair drawbacks. Many of the structures are large and custom-manufactured for the particular plant they are used in; and in the event of a structural failure, such as a ruptured water pipe, the entire plant must be shut down in order to isolate the source of the trouble, which can lead to the development of significant temperature transients. Special structures (e.g , gantries or large cranes) may also have to be assembled to repair or replace the damaged components. Finally, conventional steam generator systems often require additional auxiliary systems for decay heat removal.